Steam generation is used in heavy oil recovery operations since, in order to recover heavy oil from some geological locations, steam is necessary to increase the mobility of the oil before it can be pumped to the surface. Oil producers often use once-through (or once-thru) steam generators (OTSGs), as they are an essential part of the Steam Assisted Gravity Drainage (SAGD) processes for enhanced oil recovery of heavy oil deposits, such as from the tar sands region of Alberta, Canada. SAGD processes consist of a pair of horizontal wells drilled into the oil reservoir. Steam is injected into one of the wellbore to heat and reduce the viscosity of the oil; the oil is then collected in the second wellbore and brought to the surface. Approximately 3 barrels of water are needed for every barrel of oil produced.
SAGD processes typically make use of available brackish water in the early stages of the project to make steam. Later in the process, “produced water” (i.e., water obtained from a mixture of oil and water) is used to make steam. The OTSG uses this water after proper pretreatment to produce a mixture of steam and water which is then separated by steam-water separator vessels before the steam is injected downhole into the oil reservoir. OTSGs generally produce steam and water at a ratio of approximately 80% steam and 20% water. SAGD projects require 100% steam quality for efficient operation, so it is important to separate the water from the steam.
OTSGs typically generate steam at pressures of 8,000 to 15,000 kPa and therefore require excellent pretreatment of the water to avoid scaling and deposit problems on the heat transfer surfaces. As a result, feedwater to these boilers must typically have no more 0.2 ppm total hardness (TH) as CaCO3, a maximum of 10 ppm of hydrocarbons, preferably less than 50 ppm of silica and less than 12,000 ppm total dissolved solids (TDS). Silica can complex with calcium, iron and other multivalent cations (e.g. barium, strontium and aluminum) to form hard encrusted deposits on the boiler heat transfer surfaces, thereby reducing heat transfer efficiency and increasing the potential for rupture and failure of the boiler. The lower the concentration of hardness and other multivalent cations in the boiler water, the lower the likelihood of formation of these complex deposits.
In SAGD processes, water pretreatment can include, for example, an oil removal process, hot or warm lime softeners, and one or more ion exchange processes, often operated in series as primary and polisher vessels or as a strong acid cation resin and a weak acid cation resin vessel. Dissolved calcium, magnesium, iron, barium, strontium and aluminum are exchanged onto the strong acid cation (SAC) resin with an equivalent amount of sodium released into the treated water. Upon exhaustion (breakthrough of hardness into the treated water), a liquid brine solution is used to regenerate the SAC resin. Large quantities of salt are generally consumed in the softening of high TDS water, requiring as much as 3 barrels of water to produce 1 barrel of oil. Therefore operating costs for salt can be substantial. Large amounts of dry salt must be purchased and delivered to remote locations in many cases. Large facilities for salt storage and brine solution makeup must be installed and operated. Additionally, labor for handling of salt can be significant. For example, typical salt usage can range from 6 to 15 lbs of salt per 1000 gallons of water softened, depending on the hardness content of the produced water and the design of the softener system. For producing 33,000 barrels of oil per day, it is necessary to soften about 100,000 barrels per day of produced or brackish water, requiring between 20,000 to 50,000 pounds of salt per day, or about 9,000 tons of salt per year. At the same time, discharge of such large quantities of spent salt brine represents a substantial burden on the environment.
It is thus desirable to provide ways to minimize the purchase and use of such large quantities of salt for regeneration while maintaining or even reducing hardness leakage from the softeners.